3530
J . Am. Chem. SOC. 1988,110, 3530-3538
tungsten lamp for 10-30 min. Afterward, a measured amount of an appropriate internal standard was added, and the reaction mixtures were analyzed directly by GLC (triplicate determinations). Tables containing the experimental data used to construct the figures are available as supplementary material.
discussions and a preprint of their paper.
Acknowledgment. This research was supported by the Department of Chemistry, Virginia Polytechnic Institute and State University, and Grant 07095-19 from the National Institutes of Health. W e thank Drs. Ingold, Lusztyk, and Raner for helpful
Supplementary Material Available: Tables on the effect of neopentane and various arenes on the neopentane M / P ratios (6 pages). Ordering information is given on any current masthead page.
Registry No. DMB, 79-29-8; C6F6, 392-56-3; C6H5CF3,98-08-8; C ~ H S C N100-47-0; , (C6HS)@, 101-84-8; (C,HS)2CO, 119-61-9; C ~ H S C6H5, 92-52-4; (CH3)4C, 463-82-1.
Reactions of Alkylmercurials with Heteroatom-Centered Acceptor Radicals' Glen A. Russell,* Preecha Ngoviwatchai, Hasan 1. Tashtoush,*Anna Pla-Dalmau, and Rajive K. Khanna Contribution from the Department of Chemistry, Iowa State University, Ames, Iowa 5001 1 . Received August 24, 1987
Abstract: The relative reactivities of alkylmercury halides toward P h S , PhSe', or I' decrease drastically from R = tert-butyl to R = sec-alkyl to R = n-butyl, indicative that R' is formed in the rate-determining step in the attack of these radicals upon RHgC1. The alkyl radicals thus formed will enter into chain reactions in which a heteroatom-centered radical (A') is regenerated from substrates such as RS-SR, ArSe-SeAr, ArTe-TeAr, PhSe-S02Ar, Cl-S02Ph; Z C H = C H A (A = C1, I, SPh, S0,Ph); or Ph-HA (A = I, SPh, S0,Ph). 8-Styrenyl (PhCH=CHA, Ph2C=CHA) and 0-phenethynyl (PhC-A) systems with A = I, Br, S02Ph also enter into chain reactions with mercury(I1) salts with the ligands PhS, PhSe, PhSO,, or (EtO),PO. The relative reactivities of a series of reagents toward t-Bu' and of PhCH=CHA, Ph,C=CHA, and P h m A toward c-C6H1,' are reported as well as the regioselectivity of t-Bu' attack observed for 1,2-disubstituted ethylenes (ZCH=CHA) with Z and A from the group Ph, CI, Br, I, S02Ph, SPh, Bu3Sn. Reactions of ( E ) - and (Z)-PhCH=CHI or MeO,CCH=CHI with t-Bu' or c-C6H11*occurred in a regioselective and stereospecific (retention) manner. Reactions of ( E ) - and (Z)-CICH=CHCl occurred in a nonstereospecific manner in which the E/Z product ratio increased with the bulk of the attacking radical. A similar effect on the E / Z product ratios was observed for (Z)-MeO,CCH=CHCI.
Alkylmercurials (RHgX, R2Hg) are recognized to undergo attack a t mercury by halogen atoms to form alkyl Reactions of alkylmercurials with halogen molecules yield the alkyl halides by both homolytic attack a t mercury and electrophilic attack a t carbon although in many cases it is possible to select conditions which will favor either the ionic or homolytic p r o c e ~ s . ~ The reactions between alkylmercurials and vicinal dihalides (e.g., C2C1,) leads to dehalogenation by a free-radical chain process involving halogen atom abstraction by R' followed by a @-elimination of a halogen atom which regenerates R' by attack upon the m e r c ~ r i a l . ~ . ' The reactions of alkylmercurials with disubstituted dichalcogenides (RSSR, ArSSAr, ArSeSeAr, ArTeTeAr) have been known for some time as thermal processes,s but only recently has it been recognized that these substitutions occur by a radical chain mechanism in which the chalcogenide-centered radical attacks ( I ) Supported by grants from the National Science Foundation (CHE-
8415453) and The Petroleum Research Fund (18911-AC4-C). (2) Supported by a scholarship from Yarmouk University, Irbid, Jordan, 198c-I 983. (3) Winstein, S.; Traylor, T. G . J . Am. Chem. SOC.1956, 78, 2597. (4) Jensen, F. R.; Gale, L. H. J . Am. Chem. Soc. 1960.82, 148. Jensen, F. R.; Gale, L. H.; Rogers, J. E. J . Am. Chem. SOC.1968, 90, 5793. (5) Piotrovskii, V. K.; Bobrouskii, S . I.; Rozenberg, V. I.; Bundel, Y. G.; Reutov, 0. A. Izu. Akad. Nauk SSSR, Ser. Khim. 1975, 2285. ( 6 ) Nugent, W. A,; Kochi, J. K. J . Orgonomet. Chem. 1977, 124, 349.
(7) See, also: Werner, F.; Neuman, W. P.; Becker, H. P. J . Organomet. Chem. 1975, 97, 389. (8) Okamota, Y . ;Yano, J. J . Organomer. Chem. 1971, 29, 99.
0002-7863/88/1510-3530$01.50/0
RHgCl to form a primary, secondary, or tertiary alkyl radicaLg The free-radical chain reactions of halogens or dichalcogenides with alkylmercurials involve the attack of an electron-accepting radical (A' = 1', Cl', RS', etc.) a t the mercury atom (reaction 1). The chain reaction propagates by reaction of the alkyl radical
A'
+ RHgX(R2Hg) R' + Y-A
+
AHgX(RHgA)
RY
+ R'
(I)
+ A'
(2) thus formed with the substrate Y-A (e.g., 1-1, RS-SR). Other reagents which will react with alkyl radicals to furnish a heteroatom-centered acceptor radical which will participate in reaction 1 are benzenethiol and the arylsulfonyl halides, sulfides, and selenides. With the sulfonyl derivatives, Reaction 2 yields the arylsulfonyl radical (ArS0;) which serves as A' in reaction 1. N-Bromosuccinimide and arylsulfenyl halides might be expected to react with RHgCl by a radical chain reaction. However, electrophilic substitution processes occur so readily for these materials that no evidence for a free-radical process was observed a t 30 OC in PhH or CH2CI2solution. Another route to acceptor radicals involves the addition-elimination sequence of reactions 3 and 41° and the analogous processes ZCH=CHA
+
-
+ R'
ZCH-C(H)(R)A
ZCH-C(H)(R)A
(3)
+ A'
(4)
ZCH=CHR
(9) Russell, G. A.; Tashtoush, H. J . Am. Chem. Soc. 1983, 105, 1398. (10) Russell, G. A.; Tashtoush, H.; Ngoviwatchai, P. J . Am. Chem. SOC. 1984, 106,4622.
0 1988 American Chemical Society
J . Am. Chem. SOC.,Vol. 110, No. 11, 1988 3531
Reactions of Alkylmercurials
-
Table I. Poststimulated Reactions of Alkylmercurials with Y-A Reagents, RHgX(R) + YA RY + AHgX (or AzHg) mercurial and Y-A (equiv) conditions' % yield R-Yb CH2=CHCHzCH2HgC1and PhS-SPh (1.2) 4; D, 50 OC, 6.5; 10% DBNO, 92; 0; 0; 64 (I) 4 h; D, 10% AIBN, 80 O C , 10 CH2=CHCHzCH2HgCI and Y-A (1.2), Y-A = PhSe-SePh; PhTe-TePh; 5; 3; 4; 20 85; 92; 87; 60 (GC) PhSe-SO2C6H4Me-p;n-BUS-SBu-n CHp(CH2)4CH2HgC1and Y-A (1.2), Y-A = PhS-SPh; PhSe-SePh; 3; 4; 4; 5; 48 79; 82; 83; 82; 46 PhTe-TePh; PhSe-So2C6H4Me-p;CI-S02Ph (CH3)CCH2HgC1and Y-A (1.2), Y-A = PhS-SPh; PhSe-SePh; 12; 5; 6; 10 74; 86; 78; 75 PhTe-TePh; PhSe-S02C6H4Me-p (CHJ2CHHgCI and Y-A (1.2), Y-A = PhS-SP4 PhSe-SePh; PhTe-TePh 100; 100; 96 6; 6; 4 c-C6HIIHgCIand Y-A (1.2), Y-A = PhS-SPh; PhTe-TePh Me2S0, 16 65 (I); 72 (I) 4 c-C5H9CH2HgCIand Y-A (1.2), Y-A = PhS-SPh; PhSe-SePh 86, 73 (I); 84 6; 4; 4; 10 43f 53f 45f 48' 7-norbornyl-HgBr and Y-A (1.2), Y-A = PhS-SPh; PhSe-SePh; PhTe-TePh; PhSe-So2C6H4Me-p CH2=CH(CH2),CH2HgCland Y-A (1.2), Y-A = PhS-SPh; PhSe-SePh; 88 (GC);d93 (GC);d 88 (GC);I 3; 3; 3; 6 PhTe-TePh; PhSe-SO2C6H4Me-p 81 (GC)d CH2=CH(CH2),CH2HgC1and PhSH (1.2) 5; D, 10% DBNO, 38 58 (GC);I 0 (GC) RHgCl and PhSSPh (1.2), R = CH,(CH2)$H2; (CH,),CHCH,; (CH,),C 84 (GC); 86 (GC); 78 (GC) 6; 6; 8 2.5; 21; 2.5; 10% DBNO, [CH3(CH2)2CH2],Hgand PhS-SPh (1.2; 1.2; 2.5; 2.5; 2.5) 50 (GC); 85 (GC); 100 (GC); 1.5; D, 45 OC, 4 5 (GC); 0 (GC) 1,6-dihexanediylmercuryand PhSSPh (2.0) 2.5 95 12; 2 92 (GC); 95 (GC) [(CH,)2CHI2Hg and PhS-SPh (1.2; 2.5) 2.5; 5; 4 R2Hg and PhSe-SePh (1.2), R = n-Bu; i-Bu; 5-hexenyl 92 (GC); 90 (GC); 95 (GC)I "The mercurial (1-5 "01) in 10 mL of deoxygenated C6H6 was irradiated by a 275-W sunlamp ca. 20 cm from a Pyrex reaction vessel at 35-40 "C; D = dark; DBNO = di-tert-butylnitroxide; AIBN = azobisisobutyronitrile. The numbers not followed by a percent (%) or temperature sign ("C) designate the number of hours. *Yield determined by 'H NMR, isolation (I), or by GLC after aqueous NazS2O3/CHzCl2extraction (GC). Coupling product (7,7'-binorbornyl) observed. dMixture of 5-hexenyl and cyclopentylcarbinyl products whose ratio was dependent upon the structure and concentration of A-Y.9 involving PhCbCA," CH2=CHCH2A, or H m C H 2 A where A can be halogen or ArSO, ( n = 0, 1, 2). In the substitution processes described by reactions 1, 3, and 4, it is possible to employ mercury(I1) salts such as (PhS)2Hg, (PhS02)2Hg, [(EtO),P(O)I2Hg, or (RC02)2Hg (with decarboxylation of RCO2*to R') with I-alkenyl or 1-alkynyl derivatives, particularly the The iodine atom, or other acceptor radical formed in reaction 4, will continue the chain via ractions 5 and 6 . I'
L'
+ HgL2
+ ZCH=CHI
-
--
IHgL
+ L'
ZCH=CHL
(5)
+ I'
(6)
Reactions Involving Attack of R' upon Y-A. Table I presents a summary of yields observed in the reaction of RHgCl with a variety of Y-A reagents. Evidence for the chain sequence of reactions 1 and 2 includes the cyclization observed when R = 5-hexenyl, the photostimulation of the reactions a t 35 OC, and the inhibition by 10 mol% (f-Bu),NO' of the photostimulated r e a ~ t i o n . With ~ RHgCl the reactions do not occur in the dark a t 35 O C although slow reactions can be observed a t about 60 OC for t-BuHgC1. At this temperature the reactions are dramatically accelerated by 10 mol% of the free-radical initiation azobisisobutyronitrile (AIBN). Yields and rates of the photostimulated reactions generally increase from R = primary to secondary to tertiary alkyl. This is in part due to the rate of chain initiation, but even a t long irradiation periods, the yields with 1-BuHgC1 nearly always exceed the yields with n-BuHgC1. Further investigation led to the conclusion that in reaction 1 the reactivity of RHgX increases sharply from R = primary to secondary to tertiary alkyl. For example, under standard conditions with fluorescent sunlamp irradiation, the photodissociation of 0.25 M RHgCl in M e 2 S 0 a t 30 OC as measured by 'H N M R was 1.38 X 10" mol/L-s (0.03%/min) for R = t-Bu and 9.13 X lo-* mol/L-s (0.002%/min) for R = n-Bu.I3 With the same irradiation, reaction of 0.25 M t-BuHgC1 with 0.05 M PhSSPh formed f-BuSPh with an initial rate of 5.42 X lo4 mol/L-s leading to an initial kinetic chain length (kcl) of 400. In the presence of 0.25 M n-BuHgC1, the reaction still gave (1 1) Russell, G. A.; Ngoviwatchai, P. Tetrahedron Lett. 1986,27, 3479. (12) Russell, G. A.; Ngoviwatchai, P. Tetrahedron Lett. 1985, 26,4975. (1 3) The disappearance of RHgCl upon photolysis involves RHgCI R' 'HgCI; 'HgCI RHgCl R' HgC12 Heo. Two molecules of RHgCI may disappear for each photodissociation step.
+
+
-
+
+
Table 11. Relative Reactivities of Alkylmercury Halides toward Acceptor Radicals at 35-40 OC re1 reactivity attacking t-BuHgC1:iradical radical precursor solvent PrHnC1:n-BuHgC1 PhS' PhSSPh Me2S0 1.0:0.008: 50 observed for R = t-Bu'. The effect of added N a I on the reaction of P h C H x C H I with t-BuHgCI was investigated (Table IV). In the presence of NaI, a slow thermal substitution reaction was observed a t 50 OC, presumably initiated by electron transfer from I- to t-BuHgI or the thermal homolysis of t-BuHgIlt-BuHg1,-. Photolysis of tBuHgCl/NaI mixtures led to a somewhat faster substitution than in the absence of I- but with no appreciable effect on the stereospecificity observed for the reaction with (3-PhCH==CHI. Because of the stereospecificity observed in the photostimulated reactions of (E)- or (Z)-PhCH=CHI with RHgCI, it was possible to measure the relative reactivity of the E and Z isomers by a direct competition of a mixture of the isomers with 0.1 equiv of t-BuHgC1. The results of this experiment demonstrated that the E isomer is 1.75 times more reactive than the Z isomer toward attack by the tert-butyl radical in MezSO a t 35-40 "C. Completely regioselective attack of t-Bu' in reaction 3 with displacement of I' was observed for PhCH=CHI, Ph,C=CHI, Ph-I, PhSO,CH=CHI, MeO,CCH=CHI, and C l C H 4 HI. However, with PhCH=CHSO,Ph or P h C H e C H S P h , attack of t-Bu' occurred a t both vinyl carbon atoms. Attack a t the
PhCH=CHI
PhSOZCH=CHI ( B r )
9
C1 CH=CHI
1 I
L O
(aPhCH-CHSOZP h
C l I
.
1:2.7
PhCH-CHSPh
I'1:3' 1
PhS02CH=CHCl
r l
1~1.6
PhSOZCH=CHSPh
r 3 .
.
ClCH=CHS02Ph
T1:0.63
c%H=crph
1:1.6 1: C1 > P h S 0 2 or PhS. This trend is observed in ZCH=CHA as Z is changed from Ph to PhSOZto Cl.,O The observed selectivities probably result from a variety of factors including steric and polar effects as well as the ability of both Z and A to stabilize a radical center at the a-position by conjugation and a t the @-position by hyperconjugation or possibly bridging. As shown in Figure 1, iodine or bromine have a very strong directing effect. One might expect that &elimination of I' or Br' from the adduct radical would occur more readily than the (20) Other substituents which are effective in inducing the attack of t-Bu' at the substituted carbon atom are Bu3Sn and HgCI: Russell G. A,; Ngoviwatchai, P.; Tashtoush, H. Organometallics 1988, 7, 696.
Russell et al.
3534 J. Am. Chem. SOC.,Vol. 11 0, No. 11, 1988 Table V. Reaction of Alkylmercury Chlorides with 1,2-Disubstituted Ethylenes, Z C H d H A
+ RHgCl k R C H 4 H A + Z C H d H R % vieldb
ZCH=CHR; ( E / Z ) R C H N H A , (E/Z) conditions' RHgX (equiv) ZCH-CHA (mmoll 41 (GC); (0.5) R, 21 n-BuHgC1 (1) (Z)-ClCH=CHCI (2) 59 (Gcj; (osj R, 20 n-BuHgC1 (1) (E)-CICH=CHCI (2) 70 (GC); (0.8) R, 21 C - C ~ H I ~ H( I ~) C ~ (Z)-CICH=CHCl (2) 63 (GC); (0.7) R, 20 C - C ~ H ~ I H(1)~ C ~ (E)-ClCH=CHCI (3) 63 (GC); (41.0) R, 21 r-BuHgC1 ( I ) ( Z ) - C l C H d H C l (2) 75 (GC); (>SO) R, 20 1-BuHgC1 (1) (E)-CICH=CHCl (3) 14 (GC); (3.8) n-BuHgC1 (1) s. 7 (E)-ICH=CHI (1) 15 (GC); (8.4) C - C ~ H I ~ H(1)~ C ~ ( E ) - I C H d H I (1) s, 7 48 (GC); (>50) t-BuHgCl (1) (E)-ICH=CHI (1) s, 7 86 (NMR); (>SO) t-BuHgC1 ( 5 ) (E)-ICH=CHI (1) s, 7 C 60 (GC); (1.7) s, 10 n-BuHgC1 (2.5) (E)-CICH-CHI (1) 63 (GC); (2.3) C s, 10 C-C6HiiHgCI (1.5) (E)-CICH-CHI (1) 60 (GC); (>30) s, 10 C t-BuHgC1 (1.5) (E)-CICH=CHI (1) C 11 (GC); (2.8)d*e R, 30 n-BuHgC1 (1) (E)-ClCH=CHSPh ( I ) 8 (GC); (2.1) 49 (GC); (2.6)df R, 30 C-C~HI~H~ (1)C I ( E ) - C l C H d H S P h (1) 0%:21%. ~~
~
'
Table VI. Estimated Relative Reactivities of P h C H 4 H A toward Alkyl Radicals re1 reactivity (toward total A prod. (with t-BuHgCI) c-C6HII*) reactivitp I P h C H 4 H B u - t (100%) 1.oo 1.oo S02Ph P h C H d H B u - t (43%) 4.70b PhCH(t-Bu)CHzSOzPh (16%) 1.75') 6S SPh PhCH=CHBu-t (36%) 2.1 PhCH(t-Bu)CH,SPh and PhC(t-Bu)=CHSPh (12%) "Assuming t-Bu' and c-C6HII' have similar chemo and regioselectivities. bObserved. 'Assuming r-Bu' and c-C6HI1.have similar regioselectivities.
;:;;:)
elimination of Cl', PhS02', or PhS'. This raises the possibility that the addition of t-Bu' may be reversible, but this seems unlikely. Another possibility is that vinylic homolytic substitutions can involve a concerted addition of t-Bu' with migration of A to the 8-position or perhaps the concerted elimination of A, particularly with the iodo substituent. If such processes are involved for vinyl iodides such as P h C H = C H I , one would expect to see an increased reactivity relative to a substrate which reacted with a lower regioselectivity, e.g., PhCH PhS, HgCl > Bu3Sn, I is observed. However, in the P h C x A series, the reactivity order is P h S 0 2 (60) > I (19) > SPh (4) > SnBu3 (1). For the phenylacetylenes, the reactivity toward c-C6Hll' appears to follow the polar effect of the substituents with highest reactivity observed for attack of the electron-donating (nucleophilic) c-C6Hll'upon the system with the most powerful electron-withdrawing group.2' For the vinyl derivatives, steric and stereoelectronic considerations may be more important and a much compressed scale of reactivities (eightfold for Ph2C=CHA, fivefold for PhCH=CHA) is observed. The low reactivitiy of the vinyl iodides is noteworthy and seems to exclude any bridging effect by iodine atoms in the transition state for the regioselective addition of c-C6H11' to the 1-iodoalkenes. In SH2 substitution reactions of Y-A reagents with t-Bu', a 2000-fold range in reactivities was measured between the highly reactive PhSO2C1 and the unreactive t-BuSSBu-t. The reactivity
J. Am. Chem. Soc., Vol. 110, No. 11, 1988 3535
Reactions of Alkylmercurials
Table VII. Relative Reactivities toward Alkyl Radicals substrate A (mmol) substrate B (mmol) conditions‘ Toward Cyclohexyl Radical (Reactivity in Substitutions Only) PhSSPh (1) Ph,C=CHI (1) PhH, R, 4 h PhC=CS02Ph (1) Ph,C=CHI (1) PhH, R, 20 h Ph-I (1) PhH, R, 6 h Ph2CdHS02Ph (1) Ph-I (1) PhH, R, 6 h P h 2 C d H S P h (1) Ph,C=CHI (1) PhH, R, 6 h PhCECI (1) (E)-PhCH=CHSOzPh (1) Ph,C=CHI (1) PhH, R, 24 h PhH, S,6 h (E)-PhS02CH=CHSnBu3 Ph-I (1) Ph-I (1) PhH, R, 6 h Ph,C=C(H)HgCI (1) Me2S0, S,6 h ( E ) - M e 0 2 C C H 4 H I (1) ( E ) - P h C H 4 H I (1) P h 2 C 4 H I (1) PhH/Me2S0, R, 24 h (E)-PhCH=C(H)HgCI (1) ( E ) - P h C H e H S P h (1) Ph,C%HI (1) PhH, R, 24 h P h 2 C N H I (1) PhH, R, 20 h P h C e S P h (1) PhH, R, 5 h Ph2C=CHSnBu3 (1) P h e C I (1) P h 2 C e H I (1) PhH, R, 24 h (E)-PhCH=CHI (1) Ph,C=CHI (1) PhH, R, 24 h (E)-PhCH=CHSnBu, (1) P h 2 M H I (1) PhH, R, 20 h PhC=CSnBu3 (1) P h Z C e H I (1) Me2S0, R, 24 h ( P h e C ) , H g (1) PhH, R, 24 h CH2=CHSnBu3 (1) P h Z C 4 H I (1) P h 2 C 4 H I (1) PhH, R, 24 h CH2=CHCH2SnBu3(1)
kA/kBb
36 12.2 1.7 1.1 3.8 3.3 0.7 0.5 2.4 1.5 1.1 0.8
0.2 0.7 0.7 0.2 0.2 50), GC; 34 (1.1); 88 (>50), GC; 78 (>50) (E)-PhCH=CHS02Ph (0.1) and HgL2 (O.l), L = PhS; (EtO),PO 24; 20 50
